1. Field of the Invention
This invention relates to multilayer shaped articles including films, preforms and structures made therefrom, comprising at least one layer of poly(1,3-propylene 2,6-naphthalate) and at least one layer of poly(ethylene terephthalate). Multilayer biaxially oriented films and containers are useful in food packaging end uses.
2. Description of Related Art
Polyethylene terephthalate (PET) is widely used in the production of containers, especially beverage bottles, due to its excellent impact resistance, rigidity, gas barrier properties, light weight, and transparency. However, further improvements in gas barrier properties are desirable to increase the shelf life of products packaged in polyester bottles and films.
It has been proposed in the art to use containers composed of laminated layers for improved barrier properties. For example, multilayer bottles comprising inner and outer layers of polyethylene terephthalate, an intermediate layer of an oxygen-barrier resin, and adhesive layers interposed between adjacent layers are disclosed in Nohara et al, U.S. Pat. No. 4,741,936. Anderson U.S. Pat. No. 5,324,467 discloses oriented multilayer laminated films having at least three layers comprising polypropylene, an adhesive of a polar modified polyolefin, and a copolyester. Hosoi, et al. Japanese Kokai published patent application 5-131602 discloses laminated films comprising two resin layers, wherein each resin layer comprises a different polyester composition, and further including an intermediate tie layer comprising a copolymer of the two different polyester compositions. Preferred polyesters are poly(ethylene terephthalate), poly(ethylene 2,6-naphthalate) (PEN), and poly(1,3-cyclohexylenedimethylene terephthalate). The films are useful for high-density magnetic recording media. Use of the intermediate copolymer layer overcomes the problems of interlayer delamination and curling which can occur in certain multilayer structures, especially in flat films. The absence of this tie layer in PET/PEN multilayer films results in a lack of a adhesion between the PET and PEN layers, which leads to delamination and curling.
Matsubayashi et al. Japanese granted patent Kokoku 1-26940 discloses a polyester multilayer hollow molding made from a laminate of at least two layers where one of the layers is PET and another is PEN. The hollow moldings are obtained by blow expansion of a multilayer preform at a temperature above the glass transition temperature (Tg) of the poly(ethylene naphthalate) but below the crystallization temperature (Tc) of the poly(ethylene terephthalate). In the Examples, blow molding was conducted at temperatures between 120 and 140xc2x0 C. Because of the higher Tg of PEN (between about 113xc2x0 C. and 125xc2x0 C.) relative to the Tg of PET (between about 70xc2x0 C. and 80xc2x0 C.), the process described in the Matsubayashi et al. application requires the blow molding to be done at a temperature that is higher than the optimum processing temperature for PET, which is between about 90xc2x0 C. and 115xc2x0 C. Processing at the higher temperatures required by Matsubayashi et al. results in a reduction in strain orientation in the PET layer and a corresponding reduction in physical properties such as tensile strength. If the molding temperature was below the Tg of PEN, poor transparency and gas barrier properties resulted, as shown by Comparative Example 2. Comparative Example 3 demonstrates that when blow-molding was done at 100xc2x0 C. that there was significant thickness variation in the bottle body sidewall making blow molding impossible.
Collette et al. U.S. Pat. No. 5,628,957 teaches that in the presence of a PEN-rich layer that PET cannot be used as a co-layer in blow molding because the orientation temperature of PET is much lower than that for PEN (minimum of 127xc2x0 C.). At this temperature or above, the PET would begin to crystallize and no longer undergo strain hardening, and the resulting container would be opaque and have insufficient strength. Collette et al. overcomes the problem of the different processing requirements of PEN and PET by providing a multilayer preform and container having at least one layer of PEN which may be a homopolymer, copolymer, or blend and a core layer which comprises a non-strain-hardenable polyester such as a low-copolymer PET. The core layer can be blow molded at temperatures and stretch ratios required for enhancing the physical properties of PEN by strain orientation and crystallization. Suitable core layers disclosed in this patent include copolymers of PET and cyclohexane dimethanol (PETG), and blends of PETG and PEN.
Multilayer structures which comprise layers which can be co-stretched, for example during blow molding or film stretching processes, at temperatures that provide improved mechanical and barrier properties, and which do not require an intermediate adhesive tie layer, would represent an improvement over those disclosed in the art.
The present invention overcomes the problems described above by providing improved multilayer shaped articles such as films, preforms, and hollow containers comprising at least a first resin layer and a second resin layer wherein the first resin layer comprises poly(1, 3-propylene 2,6-naphthalate) (3GN) and the second resin layer comprises poly(ethylene terephthalate).
Multilayer films and preforms of the current invention can be biaxially stretched at optimum PET processing temperatures (90-115xc2x0 C.) to provide oriented films and other structures such as bottles having excellent mechanical, optical, and barrier properties. In addition, the multilayer films have excellent adhesion between the 3GN and PET layers, without problems of delamination and curling and do not require an adhesive tie layer between the 3GN and PET layers.
One or more additional resin layers can be present wherein the additional layers comprise 3GN or PET or other polymeric compositions which can be co-stretched with 3GN and PET at temperatures between about 90xc2x0 C. and about 115xc2x0 C.
In a process for forming oriented films, a substantially amorphous multilayer 3GN/PET film is formed, for example by coextrusion of the 3GN and PET layers, followed by stretching uniaxially or preferably biaxially at a temperature between about 90xc2x0 C. and 115xc2x0 C.
The films are generally heat set at a temperature of about 160xc2x0 C. to about 180xc2x0 C. after stretching.
Hollow containers, such as bottles, can be formed by first forming a multilayer 3GN/PET preform followed by blow molding at a temperature of from about 90xc2x0 C. to about 115xc2x0 C. Other methods known in the art which do not require formation of a preform to form multilayer shaped articles can also be used.